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Author: MULAN -Plastic Molding Manufacturer
During production, when the plastic melt is injected into the mold cavity under high temperature and high pressure, and is molded under pressure, when the temperature drops, the melt cools and solidifies into a plastic part. The size of the plastic part is smaller than the size of the mold cavity, called shorten. The main reasons for the shortening are as follows. When making plastic, the cross-sectional dimensions of different mold gates are different.
The large gate helps to increase the cavity pressure, prolongs the closing time of the gate, and facilitates more melt to flow into the cavity, so the density of the plastic part is also higher, thereby reducing the shrinkage rate, otherwise it will increase the shrinkage rate. Changes in chemical structure during plastic mold making. Some plastics change their chemical structure during the molding process.
For example, in thermosetting plastics, the resin molecules change from a linear structure to a body structure, and the volume mass of the body structure is greater than that of the line structure, so its total volume shrinks, resulting in shrinkage. Thin-wall plastic parts with uniform wall thickness cool faster in the mold cavity, and the shrinkage rate tends to be the smallest after demoulding. The longer the cooling time of thick plastic parts with the same wall thickness in the cavity, the greater the shortening after demolding.
If the thickness of plastic parts is different, there will be a certain degree of shrinkage after demoulding. In the case of such a sudden change in wall thickness, the shortening rate will also change suddenly, resulting in greater internal stress. Residual stress changes.
When plastic parts are molded, due to the influence of molding pressure and shear force, anisotropy, uneven mixing of additives and mold temperature, there will be residual stress in the molded plastic parts, and the residual stress will gradually become smaller and redistributed, resulting in plastic parts The further shortening is generally called post-shortening.